The present invention relates to a variable black level bias image display for video projectors and television receivers and, more particularly, to an image display, in which a reference signal is superimposed on an image signal in the horizontal or vertical fly-back time for feed-back control of the beam current in the CRT cathode by detecting the beam current in the reference signal period and also for a black level bias voltage control according to the signal level of the reference signal.
A prior art image display of the pertaining type has a circuit, which superimposes a reference signal on an image signal in the horizontal or vertical fly-back time for controlling the grid G1 or G2 or cathode of a CRT such as to make the cathode beam current therein constant by detecting the beam current in the reference signal period, as well as for brightness control.
FIG. 6 is a block diagram showing a prior art image display circuit. FIG. 7 is a waveform chart showing waveforms appearing in various parts of the circuit shown in FIG. 6.
An image signal is inputted to an input terminal 1. A reference signal generator 4 outputs a reference signal during the horizontal or vertical fly-back time. An adder 5 superimposes the reference signal on the image signal. An image amplifier 6 amplifies the output of the adder 5, i.e., the image signal with the reference signal added thereto. The amplified image signal is coupled through a beam current detector 7 to the cathode of a CRT 8.
The beam current detector 7 detects a beam current flowing through the cathode of the CRT 8, and converts the detected beam current into a corresponding voltage. The converted voltage is fed to a sample-hold circuit 9. The sample-hold circuit 9 samples the converted voltage at the timing of the input of the reference signal from the reference signal generator 4, and holds the sampled voltage until the input of the next reference signal.
The sampled and held voltage is fed to an inverted input terminal of a voltage comparator 11. A reference voltage 10 is applied to a non-inverted input terminal of the voltage comparator 11. The voltage comparator 11 compares the voltages at the inverted and non-inverted input terminals, and feeds a voltage corresponding to the difference to a bias amplifier 12. The bias amplifier 12 feeds an output signal to the grid G2 of the CRT 8.
The operation of the above image display circuit will now be descried.
The reference signal provided from the reference signal generator 4 is added to the input image signal. The resultant image signal is amplified in the image amplifier 6 to a prescribed voltage level. The amplified signal is coupled through the beam current detector to the cathode of the CRT 8. A beam current corresponding to the coupled signal is caused to flow through the cathode of the CRT 8. The beam current detector 7 detects the beam current and converts the detected beam current into a voltage. The converted voltage is fed to the sample/hold circuit 9. The sample/hold circuit 9 samples the converted voltage in the reference signal period in correspondence to the timing of the reference signal. The sampled voltage is held until the input of the next reference signal. The voltage that is held is fed to the inverted input terminal of the voltage comparator 11.
The reference voltage 10 is applied to the non-inverted input terminal of the voltage comparator 11. The voltage comparator 11 provides the difference voltage to the bias amplifier 12. The bias amplifier 12 feeds an output signal to the grid G2 of the CRT 8.
When the sampled voltage is higher than the reference voltage 10, the voltage comparator 11 reduces the output voltage, thus reducing the output of the bias amplifier 12 to reduce the beam current. On the other hand, when the sampled voltage is lower than the reference voltage 10, the voltage fed to the grid G2 is increased to increase the beam current. It will be seen that the voltage comparator 11 functions to make the sampled voltage to be equal to the reference voltage.
FIGS. 8(A) and 8(B) show the relation between the cathode voltage amplitude and the brightness when the amplitude of the reference signal in the above image display circuit is controlled. By increasing the amplitude of the reference signal, the image is caused to sink to the black level side. By reducing the amplitude of the reference signal, on the other hand, the image floats up to the white level side. In this way, the brightness control is obtained.
The above prior art image display had the following problem.
Referring to FIGS. 8(A) and 8(B), when the detected level of the reference signal is set to the vicinity of the black level (i.e., cut-off level) (FIG. 8(A)) by reducing the reference voltage 10 for making the reference signal to be dark, the maximum brightness point on the screen is one, at which the reference signal coincides with the image signal pedestal level. Since the image signal pedestal level is substantially the same as the black level, no back luster float-up can be obtained, and the brightness range is insufficient.
When the detected level of the reference signal is set to be on the white level side more than the black level (FIG. 8(B)) by increasing the reference voltage 10, the maximum brightness at this time is one, at which the image signal pedestal level is on the white level side more than the black level. Thus, the back luster float-up can be obtained, and the brightness range is sufficient. However, since the detected level of the reference signal is shifted to the white level side, the reference signal provides a bright image.
That is, by making the image brighter, i.e., providing a sufficient brightness range, the reference signal provides a considerably bright image. This is so for the following reason. To detect the beam current, it is necessary to cause the beam current to flow through the CRT 8. By causing the beam current, the CRT 8 becomes bright. This is why the reference signal provides a bright image.
Well-known image displays are disclosed in Japanese Laid-Open Patent Publication No. 56-007575 and Japanese Laid-Open Utility Model Publication No. 56-114171 (both published on 1981). These techniques seek to suppress the CRT beam current when an excessive input signal appears. To this end, the amplification factor concerning the image signal or the clamp level is controlled. Therefore, in these cases the reference screen brightness or shade is varied. For this reason, it has been inevitable to adopt the image display circuit as described above for permitting brightness control by holding fixed brightness or shade.